1. Field of the Invention
The present invention relates to electrical power supplies which derive electrical energy from a low frequency medium voltage source (e.g. a 120 V 60 Hz ac main), and which convert the electrical energy to another format suitable for the assigned load. More particularly, the invention relates to an improved converter for use in such supplies and to a combination of converters particularly adapted for providing low voltage electrical power to solid state signal processing equipment.
2. Description of the Prior Art
The primary function of an electrical "power supply" is to convert the electrical power in the available form to a form suited to a particular apparatus. The present power supply is designed for solid state electronic circuitry which requires a low supply voltage typically between 4 and 20 volts. The power levels of such supplies frequently lie between 50 and several hundred watts.
There has been a growing need for size and weight reduction in power supplies for solid state electronic circuitry. The continuing technological evolution has caused a continuous shrinkage in the size and weight of signal and data processing hardware, and the percentage of equipment cabinet volume and the percentage of equipment weight for the power supply function has continued to increase.
In addition to the need for reduced size and weight, a power supply for signal and data processing should have reasonable efficiency, high reliability and reasonable EMI (electromagnetic interference) performance.
Another feature of importance, particularly in supplies where significant amounts of power are drawn from the power line, is that the input power factor should be inductive, preferably greater than 0.8, and that the current drain should be continuous.
A final feature of critical importance to such supplies, is that the supply should have a substantial voltage "hold-up" after loss of input power. If this hold-up can be maintained for a reasonable period, periods of up to 50 milliseconds being useful for this purpose, the loss of signal information in processing or storage may be prevented.
A well-known approach to power supplies for semiconductor applications is to use a bridge rectifier to derive a dc voltage, followed by a capacitor bank to obtain an averaged dc output, followed by a converter, frequently of the half-bridge variety, to obtain an ac voltage. The latter ac voltage is then transformed by a transformer to a desired output voltage, which is then rectified and coupled to the load. Compactness and weight reductions in respect to conventional 60 hertz supplies have resulted from the use of converters operating in the 18 to 25 KHz region. The benefits of using higher frequencies to achieve greater compactness and additional weight reductions is accordingly acknowledged. However, in critical applications, the full complement of performance features must be dealt with.
The organization of the conventional converter is not generally satisfactory for such critical applications. In the conventional supply, rectification is followed by a capacitor energy storage bank, and the latter is followed by a converter. The presence of the capacitor storage bank for energy storage in such power supplies immediately following ac line rectifiers produces peak rectification and line current conduction in narrow pulses. Conduction in narrow pulses generates harmonics and thus distortion in the line current and a reduced power factor, all of which are objectionable. These objections have been avoided in the past by using a 12 pulse .DELTA.-Y 60 hertz transformer connected ahead of the capacitor bank. Such a transformer occupies too much space for consideration under the foregoing constraints, and forces the adoption of a converter design in which there are no large 60 hertz inductive components.